Session: 27-05 Non-Linear Rotordynamics

Paper Number: 122018

122018 - Application of Nonlinear Frequency Response Analysis for Windmilling 

A windmilling event causes an unbalanced force on the rotating part of the engine for the duration of the rest of the flight. Even when shut down, the engine rotates due to airflow. Typically windmilling will occur after a blade-out event. The Federal Aviation Authority (FAA) requirement is to demonstrate either by test or analysis, that the aircraft can land safely at the nearest airport taking the diversion profile. During the windmilling event, the rotor may rub against the casing and the bearings often show nonlinear behavior. The current best practice across the aero industry is to model this event in a time-domain analysis. This can be computationally expensive, especially for large engine models and does not provide information on all the possible stable non-linear forced solutions.

Frequency response analysis provides great deal of information about the system response over the entire range of operation. However, the presence of non-linearities in the system make it difficult to employ standard frequency response analysis techniques which are linear in nature. If the system contains mild-nonlinearities and the response of the system can be assumed to be periodic (which is usually the case for aero engines), it is possible to obtain the nonlinear frequency response of the system using harmonic balance techniques. Nonlinear harmonic response uses an iterative procedure to find the coefficients for the combination of sinusoids that form the steady-state response. Newton’s method of iteration is employed to solve a system of nonlinear algebraic equations. In addition to the Harmonic balance technique, the continuation procedure method is required in the frequency response solver to capture the complete solution that consists of stable and unstable branches. 

This paper will present the application of the harmonic balance method with a continuation procedure for solving nonlinear rotordynamics problems. The techniques developed are then used to determine the response of a generic aero-engine model to unbalance loads in the presence of nonlinearities in the bearings. To make it possible for the analysis to work on large existing models, this method has been implemented in MSC Nastran. It involves enhancing the existing SOL 128 solution sequence in MSC Nastran. Several new entries have been added in Nastran to facilitate the use of the new solution method. Using the reduction techniques in MSC Nastran, it is possible to reduce the engine model with hundreds of thousands of DOFs to a few hundred DOFs which are used for the nonlinear analysis. This paper demonstrates the application of this method on a small engine test and for a generic engine model. The application of the process developed is not limited to windmilling, it can be applied to any phenomenon that results in non-linear bearing stiffness and damping properties, like rotor-stator rub and oil film whipping.

Presenting Author: Devesh Kumar Boeing

Presenting Author Biography: His expertise is in structural dynamics, finite element modeling, analysis of rotating systems, helicopter dynamics and design optimization.
He graduated with a doctorate degree in Aerospace Engineering from the University of Michigan, in 2013. He is a member of AIAA’s Structural Dynamics Technical Committee and reviewer for many peer-reviewed journals.

Authors:

Devesh Kumar Boeing
Don Powell The Boeing COmpany
David Carlson CTS TECHNICAL SERVICES INC
J S Kumar Hexagon MI
Jianming Cao Hexagon MI